Rome even in her decline did not build her citadels outside the walls: that was a habit which grew up in the Dark and early Middle Ages, and was attached to the differentiation between the civic and military aspects of the State. Again, Roman fortification of every kind is connected with earthworks. So far as we can tell from recorded history the ditch round the Tower was not dug till the end of the twelfth century. Finally, there is this strong argument against the theory of a Roman origin to the Tower that had such a Roman fortress existed an extension of the town would almost certainly have gathered round it. One of the features of the break-up of Roman society was the enormous expansion of the towns. We have evidence of it on every side and nowhere more than in Northern Africa. This expansion took place everywhere, but especially and invariably in the presence of a garrison, and indeed the military conditions of the fourth century, with its cosmopolitan and partially hereditary army, fixed in permanent garrisons and forming as it were a local caste, presupposed a large dependent civilian population at the very gates of the camp or stronghold. Thus you have the Palatine suburb to the south of Lutetia right up against the camp, and Verecunda just outside Lamboesis. Now there is nothing of the sort in the neighbourhood of the Tower. It seems certain that from the earliest times London ended here cleanly at the wall, and that except along the Great Eastern Road the neighbourhood of the Tower was agricultural land. How then could a tradition have arisen with regard to Roman occupation? It is but a conjecture, though a plausible one, that when the pirate raids grew in severity this knoll down stream was fortified, while still the ruling class was Latin speaking and while still the title of Cæsar was familiar, whether before or after the withdrawal of the Legions. If this were the case, then, on the analogy of other similar sites, one may imagine something like the following: that in the Dark Ages the masonry was used as a quarry for other constructions, that the barbarians would occasionally stockade the site, though not permanently, and only for the purposes of their ephemeral but constant quarrels; and one may suggest that when the barbaric period was ended, by the landing of William's army, the place was still, by a tradition now six hundred years old, a public area under the control of the Crown and one such as would lend itself to the design of a permanent fortification. William, finding it in this condition, erected upon it the great keep which was to be the last of his fortifications along the line of the river, and the pivot for the control of London.